In general, laminated sheets to be used as wrapping materials are required to have various characteristics depending on the type, shape, weight and so forth of an article to be wrapped. In particular, a wrapping material for use in the preparation of a bag is required to have sufficient physical strength, e.g., tear strength, tensile strength, burst strength, and impact strength, and if necessary, good cushioning characteristics.
Laminated sheets for use as construction materials, particularly floor materials and wall materials must have good cushioning characteristics, surface strength, and printing suitability, and furthermore, be inexpensive.
Such laminated sheets, particularly those for use as wrapping materials have heretofore been known as described in U.S. Pat. Nos. 4,147,291 and 4,258,848. These laminated sheets have a fundamental layer construction as illustrated in FIG. 1, in which two uniaxially drawn high density polyethylene films 1 and 2 are disposed in such a manner that the drawing axes of the two films form an angle of 45.degree. and 90.degree., and are bonded together through an adhesive layer 3. The adhesive layer 3 is usually made of low density polyethylene. In some cases, however, in place of the provision of the adhesive layer 3, the films 1 and 2 may be heat-sealed at a suitable temperature, e.g., about 180.degree. C.
These laminated sheets have high strength in any of longitudinal, lateral and oblique directions because the drawing directions of the films cross with each other. They are, therefore, greatly improved in the foregoing physical strength, at least, compared with the conventional composite laminated sheets, for example, they are stronger than those sheets composed of a low density polyethylene film, and paper, an aluminum foil or the like.
It has been revealed, however, that the physical strength of the laminated sheet varies depending on the thickness of the adhesive layer 3, i.e., as the thickness is increased, the physical strength is reduced and the cushioning characteristics are deteriorated. The reason for this is believed that when the films 1 and 2 are bonded together with the adhesive layer 3 having a thickness exceeding a certain critical level, they are united completely as one body, and act as substantially one film even though the two films are adhered in such a manner that the drawing axes of the two films intersect at a certain angle. It is, therefore, necessary to reduce the thickness of the adhesive layer 3 to as low a level as possible. However, when the thickness of the adhesive layer 3 is reduced excessively, problems such as formation of film cutting and stripping-apart of the films 1 and 2 due to a reduction of adhesion force therebetween arise.
The method of heat-sealing the films 1 and 2 is also not desirable in that heat shrinkage of the film is caused at areas where heating is applied, resulting in the formation of wrinkles which is not desirable from a viewpoint of quality.